1. Technical Field
The present invention relates to a method and a device for producing embossed sheet material consisting of a plurality of layers, and to a sheet product made in this way.
2. Prior Art
In the manufacture of paper sheet products for domestic use and similar, a process of embossing a paper web material is frequently carried out to produce a greater apparent thickness, good characteristics of liquid absorbency, characteristics of softness to the touch and a decorative effect.
The embossed sheet web material is used for the production of kitchen paper, toilet paper, serviettes, paper tissues, and similar. In general, this web material consists of two or more layers which are usually embossed separately from each other and subsequently joined, usually by means of an adhesive or by other methods, such as ply-bonding. Sheet products having great softness and thickness, and good liquid-absorption characteristics, are produced in this way.
The embossing and the joining of two or more layers normally takes place by two methods, called xe2x80x9ctip-to-tipxe2x80x9d and xe2x80x9cnestedxe2x80x9d joining. In the first case, two layers of embossed material are joined by lamination between two embossing cylinders with parallel contra-rotating shafts. The two cylinders are provided with points which coincide, at least partially, in a lamination nip formed at the point of tangency between the two cylinders. An adhesive is applied to the protuberances of one of the two embossed layers to provide permanent joining to the other layer at the positions of the protuberances of the other layer in the areas in which the points of the two embossing cylinders coincide with each other. Examples of embossing machines of this type are described in U.S. Pat. No. 3,414,459, U.S. Pat. No. 4,978,565, U.S. Pat. No. 5,173,351, U.S. Pat. No. 5,096,527, U.S. Pat. No. 3,961,119, WO-A-9720687, WO-A-9720688, WO-A-9720689.
In other devices, the layers are joined in such a way that the protuberances of one layer are nested in the cavities between adjacent protuberances of the other layer. In this case, the two embossing cylinders are not pressed against each other at the corresponding points, and the two layers are joined together by lamination by means of a pressure roller which interacts with the surface of one of the embossing cylinders, on which both layers are supported as they emerge from the nip between the embossing cylinders.
Examples of this type of embossing are described and illustrated in GB-A-1,225,440 and U.S. Pat. No. 3,694,300.
Normally, the embossing, whether of the tip-to-tip or the nested type, consists of a geometrical and uniform distribution of protuberances, typically of frusto-conical or frusto-pyramidal shape, on the two layers. To obtain a product with a more attractive appearance, systems in which protuberances of dissimilar shapes are combined with each other to obtain a particular design have been devised. For example, U.S. Pat. No. 4,320,162 describes an embossing system in which embossing consisting of a geometrical and uniform distribution of small protuberances arranged with a high density, forming a fine background embossing, combined with a distribution at low density of protuberances of complex shape and of large size, forming a decorative motif, is produced on each of the two layers. Similar product is described in WO-A-9618771.
A disadvantage of this system consists in the fact that when it is desired to change the decorative motif it is necessary to make a new pair of embossing cylinders, or at least one new embossing cylinder, and this requires the new production not only of the whole of the decorative motif but also of all the points of the cylinder which produce the background embossing.
The object of the present invention is to provide a method and a device for producing an embossed product of the type comprising a first embossing and a second more widely spaced embossing consisting of decorative motifs, in a similar way to that described in U.S. Pat. No. 4,320,162, which provides a high quality.
A further object of the present invention is to provide an embossing device and method which enable the decorative motifs to be modified in an economical way.
Yet another object of the present invention is to provide a method and a device which enable a soft product to be produced with the possibility of increasing the decorative effect of the embossing by combining it with the use of colored adhesives.
It is also an object of the invention to provide an embossing device which can be produced in an economical way by the conversion of existing embossing systems.
These and further objects and advantages, which will be evident to persons skilled in the art from a reading of the following text, are achieved according to the invention by means of a process comprising the stages of:
embossing a first layer of web material previously furnished with a background pattern made up of a first set of protuberances, so as to generate on it a second set of protuberances which are partially superimposed on the first set of protuberances and of larger dimensions and lower density than the protuberances of said first set of protuberances;
to couple, preferably by means of gluing, a second layer to the so embossed first layer.
The second layer may be smooth, embossed with a background pattern, embossed with an ornamental motif or embossed both with a background pattern and an ornamental motif.
In an especially advantageous embodiment of the invention, the protuberances of the second and third sets of protuberances, arranged on the first and second layers respectively, are generated by the same embossing cylinder with which two pressure rollers interact, so that they exhibit the same density and the same arrangement and lie inside one another. In this way it is also possible to unite the two layers by the application of an adhesive to the protuberances of the third set of protuberances present on the second layer, and then to laminate the first and second layers in the course of generating the second set of protuberances. The adhesive may be applied to all protuberances of the third set or only to some of these, as is known per se.
In a practical embodiment of the method according to the invention the first and second sets of protuberances can be generated on the first layer of web material by running the latter around a first pressure roller which interacts with a first and a second embossing cylinders. The two embossing cylinders comprise respectively first and second sets of points, the second set of points being of larger dimensions and lower density than the first set of points.
The second embossing cylinder may interact with a second pressure roller to generate the third set of protuberances on the second layer.
The embossing on two layers may be such as to generate the protuberances of the first and third sets with approximately equal depth, that is to say height. Alternatively the height of the protuberances of the third set may be made greater than the height of the protuberances of the second set. In this way, joining the two layers generates hollow spaces which enhance the softness and absorption capacity of the web material.
In another embodiment of the method according to the invention, the second layer may be run around the first pressure roller with which the first and second embossing cylinders interact. Contact between the second layer and the first pressure roller occurs downstream of the area in which the first layer is embossed, between the first embossing cylinder and the first pressure roller. By this means the second layer is embossed on the first layer between the first pressure roller and the second embossing cylinder which interacts with the first pressure roller.
Another possibility involves arranging a second pressure roller around the second embossing cylinder and feeding a third layer of web material around the second pressure roller, between the latter and the second embossing cylinder. The third layer of web material is then embossed by the generation upon it of a fourth set of protuberances; the first, second and third layers are then laminated together between the second embossing cylinder and the second pressure roller. The layers can be united with an adhesive which is applied to at least some of the protuberances of the second set of protuberances produced on the first layer. The gluing of the three layers occurs by the migration of the adhesive through the second layer toward the first layer.
One advantageous embodiment of a device for carrying out the method according to the invention involves using a pressure roller interacting with a first and a second embossing cylinders carrying respective points on their cylindrical surfaces. The two embossing cylinders preferably have points with dissimilar densities, and in particular with larger dimensions and lower density on one and lower density and larger dimensions on the other.
According to a different form of embodiment of the invention, there is provided a process comprising the stages of:
carrying out a first embossing on the two layers separately, according to a background pattern formed by a first set of protuberances;
re-embossing at least one of the two layers with an ornamental motif having larger dimensions and a lower density than those of said background pattern, formed by a second set of protuberances which are partially superimposed on protuberances of the first set of protuberances of said background pattern; and
joining said two layers.
In practice, the protuberances of the background pattern and the protuberances of the ornamental motif may be made to project on the same face of the corresponding layer, particularly that which, in the final jointed product, faces the interior of the product.
In a preferred embodiment of the invention, the protuberances of the second set, forming the ornamental motif, have a greater height than that of the protuberances of the first set which form the background pattern, and the two layers are joined together by gluing at the positions of the protuberances of the second set.
To obtain a particular appearance, it is possible, according to a possible embodiment of the process, to arrange for the layers to be joined together by means of a colored adhesive, so that the ornamental motif stands out from the background of the web material.
The device for producing a web material with at least two embossed and joined layers comprises, according to the invention,
a first embossing unit for a first layer and a first embossing unit for a second layer, said first embossing units generating in said first and said second layers a background pattern consisting of a first set of protuberances;
a second embossing unit for generating, in the first layer, an ornamental motif consisting of a second set of protuberances having a density lower than that of the first set of protuberances, which are partially superimposed on said background pattern; and
joining and laminating members for joining together said first and said second layers.
Advantageously, an adhesive dispenser associated with said second embossing unit is provided, to apply an adhesive to at least some of the protuberances of said first layer, and then to carry out the joining together of the two layers by gluing and lamination.
In a particularly advantageous embodiment of the device, the joining and laminating members join the two layers at the positions of the protuberances of the ornamental motif, which for this purpose have a greater height than the protuberances of the background pattern.
In a possible embodiment of the device, each of the first two embossing units for the first and the second layers comprises a pair of embossing rollers, one of which is provided with points while the other is provided with a yielding surface; alternatively, each (or at least one) comprises a pair of rollers made from steel or other hard material.
The joining and laminating members consist, for example, of an embossing cylinder of the second embossing unit and a marrying roller, said embossing cylinder being provided with points for generating the second set of protuberances forming the ornamental motif.
In a different embodiment of the device according to the present invention, the first embossing unit for the first layer comprises a pair of embossing rollers; the second embossing unit comprises an embossing cylinder, provided with points for generating the second set of protuberances forming said ornamental motif, and interacting with a pressure roller; and the first embossing unit for the second layer comprises a further embossing cylinder provided with points and interacting with a pressure roller having a yielding surface. In this case, the joining and laminating members may consist of the embossing cylinder of the second embossing unit for the first layer and of said further embossing cylinder for the second layer, which form between them a lamination area in which the points of the two embossing cylinders interact to join the two embossed layers.
The invention also relates to a sheet material formed by at least two embossed and joined layers, characterized in that the first of said layers comprises a background embossing consisting of a first set of protuberances and a second embossing formed by a second set of protuberances forming an ornamental motif, the protuberances of said second set having larger dimensions and a lower density than said first set of protuberances, the protuberances of the second set being superimposed on the protuberances of the first set; and in that said second layer comprises at least one background embossing consisting of a set of protuberances.
Further advantageous characteristics and embodiments of the method, the device and the product according to the invention are described below and indicated in the attached dependent claims.